Effect of Substituents on the Structure of the Vinyl Radical:  Calculations and Experiments

Abstract

Literature reports indicate either a linear or a bent configuration for α-substituted vinyl radicals. In order to get a better insight into this structural question, calculations of the structure of some α-substituted vinyl radicals were conducted by the DFT approach [BLYP/6-31G(d,p) and B3LYP/6-311G(2d,2p)]. For vinyl radicals bearing σ-type substituents (Me, SH, Cl, OH, F), the bent form is found to be the minimum energy structure; the inversion barrier of the E and Z forms is found to markedly increase as the electronegativity of the group increases. Vinyl radicals bearing π-type substituents (CHCH2, CHO, CN, C6H5) are found to be linear. The effect of β-substituents is much lower, and the β-fluoro- and β-ethenylvinyl radicals, representative of σ- and π-type substituents, are calculated to be bent. Comparison with other literature calculations, obtained by different methods for some similar vinylic radicals, supports our computational results. Experimental support was also sought, with particular regards to the inversion barrier of the bent radicals. Since a very high barrier is calculated for the α-fluorovinyl radical, its inversion rate could be slower than the “sampling” time of a proper chemical reaction. Indeed, the α-fluorovinyl radicals, generated from PhCHC(F)Br by reaction with Bu3SnH/AIBN, retained the configuration of their precursors in the overall hydrodebromination process.

Reactivity of Aryl and Vinyl Radicals: Abstraction of Hydrogen Atom or Reaction with a Nucleophile

“…The phenyl radical accommodates the odd electron in an sp 2 orbital perpendicular to the π system of the benzene ring, and consequently no delocalization of it is possible. Vinyl radicals possessing π-type substituents α to the carbon atom bearing the odd electron, on the other hand, have been found to be linear (i.e., with sp hybridization [11,27] ). For such vinyl radicals, the π system of the substituent is orthogonal to the vinylic π system, but colinear with the p orbital hosting the odd electron.…”

Kinetics and thermochemistry of the addition of atomic chlorine to acetylene

“…Although needed for reliable combustion modeling, no experimental data are available for the enthalpies of formation of any chlorovinyl radicals. These have been the subject of prior computational studies [9,12,[14][15][16].…”

Radical Additions of Arenethiols to Ynamides for the Selective Synthesis of N-[(Z)-2-(Arylsulfanyl)-1-alkenyl]amides

“…11 Addition of the phenylsulfanyl radical thus naturally takes place at the more electron rich sp-hybridized carbon of ynamide 1a. The newly formed nitrogen-substituted vinyl radical 3 should have Z geometry, 12 and the isomerization of (Z)-3 into the E isomer (E)-3 would be so slow because of the electronegative nitrogen 13 that (Z)-3 abstracts hydrogen atom from benzenethiol. Product 2a is thus selectively formed.…”

“…1 H NMR (500 MHz) and 13 C NMR (125.7 MHz) spectra were taken on a Varian UNITY INOVA 500 spectrometer and were obtained in CDCl3 with tetramethylsilane as an internal standard. IR spectra were determined on a SHI-MADZU FTIR-8200PC spectrometer.…”